THE EARTH'S CORE

The inner core - remnant of a planet's birth

Holding the secrets of our planet's formation, the Earth's core is something of a "holy grail" for the geosciences. The exact composition of its liquid and solid components remains a mystery, as do the complex phenomena that occur there. Located at a depth of over 3 000 km, its inaccessibility obliges researchers to be inventive. Only one thing is certain: the inner core that constitutes its heart is crystallising, thereby heralding - albeit in the very long term - the likely disappearance of our protective magnetic field.

Even in 1864, in his novel Journey to the centre of the Earth, Jules Verne imagined travelling there through the mouth of an Icelandic volcano.

In 2003, the prestigious geologist David Stevenson of the California Technol ogy Institute (USA) was himself bordering on science fiction when he suggested producing a gigantic explosion to create a crack in the Earth's crust into which molten iron would be poured. This would contain a probe that, through the force of gravity, would descend to the centre of the Earth from where it would send back messages. Mysterious and impenetrable the centre of the Earth continues to arouse the interest of scientists as they compete in their ingenuity in seeking to reveal, at a distance of 5 000 km, the composition of its hard inner core.

Beneath the Gaia mantle

Let us travel back in time to the birth of our marvellous blue planet. Formed 4.5 billion years ago along with the rest of the solar system, the Earth results from the coming together of molten celestial bodies. Given its proximity to the Sun, surface temperatures - of between 800 °C and 1 300°C - enabled this liquid matter to amalgamate while retaining a rotational movement that caused the planet's spherical shape. During a differentiation phase, heavy particles such as iron or nickel (1) descended deeper into the molten rock to constitute the core, surrounded by a mantle of lighter elements - the silicates. The continental and oceanic crusts formed later, after the surface temperature cooled.

We believe these crusts to be between 35 and 70 km thick. To a depth of 2 885 km below that lies the mantle with varying components depending on the depth. You therefore have to descend to a depth of almost 3 000 km to reach the liquid core and to over 5 000 km to arrive at the hard inner core.

Given these vast distances, how have geologists managed to distinguish these different layers and formulate reasonable hypotheses as to their composition? "There are a number of methods for analysing the terrestrial abysses," explains Véronique Dehant, former head of the Special Bureau for the Core and Head of Department at the Belgian Royal Observatory. "For example, one can refer to the structures of metal asteroids that, when they were formed under conditions similar to those at the origin of the Earth, provide a vast amount of information. But among all the available techniques," she adds, "seismology is without a doubt the most effective in studying the Earth's core."

Earthquakes reveal all

Telluric seisms are the result of the combination of two phenomena - compression and shearing. The speed at which the waves - known as P (primary) and S (secondary) waves - travel depends very much on the composition of the ground they cross. When an earthquake occurs, the shock waves leave from the epicentre to be reflected against the Earth's internal interfaces or even refracted or diffracted. "First the P wave and then the S wave arrive. The lower the density of the environment, the greater the speed at which they travel, although this also depends on rheological parameters particular to these two wave types. Knowledge of the speed at which these waves travel is therefore also a way of identifying the structure of the different layers of matter."(2)

It was as a result of this seismic technique that the complex structure of the Earth's core was first revealed in 1906. After comparing all the seismic readings in their possession, scientists noticed that each seism corresponded to a "grey" area in which no shearing wave emerges from the ground. This observation suggested that the state of matter at the centre of the Earth stops the propagation of waves of this type - hence the notion of a liquid core. However, it was not unusual for P waves to cross the entire planet with notable variations in speed close to the centre of the globe. Just one model could explain this dual effect: the core consists of an outer layer that remains in liquid form due to the effect of heat - 4 000 - 5 000 °C - and an inner layer that, due to the effect of increased pressure at this depth, has become solid over time. This is the inner core.

A crystallising inner core

Contrary to geologists' explanations, iron and nickel are not the only components of this inner core, as on the basis of the data collected to date the density suggests the presence of lighter elements such as sulphur and/or oxygen (1). If these lower-density substances were not present, and given the temperature and pressure conditions at the heart of the planet, a single iron/nickel (Fe/Ni) core would be entirely solid. Yet the Earth's magnetic field is the result of internal convection movements by the liquid part of the core. Without it, this field that protects us from the solar wind and renders our planet habitable would most probably disappear.

It is the presence of these light elements that delays the core solidification. "Knowing the exact nature of this alloy is very important for understanding the Earth's development.At present, the inner core is crystallising through Fe/Ni precipitation. But in so doing the liquid core is being impoverished and we believe that this continuous change in the composition of the liquid core will ultimately cause a shift in the phase diagram of the mixing. Beyond a certain ratio of heavy to light elements, precipitation stops and a direct physical transfer occurs from the liquid to solid state."

There is continued interest in studying the nature of the Earth's core because it no doubt determines the presence or otherwise of life. "Water disappeared from Mars 3.5 billion years ago. Probes have shown that a large part of the atmosphere escaped whereas originally this close neighbour of Earth was probably habitable. Without atmosphere, the pressure is so low that it enables water to move directly from the solid to the gaseous state, the liquid phase needed for life being non-existent. Whether by chance or coincidence, Mars also lost its magnetic field at about the same time.So there are convincing reasons for believing that the core is a key player in a planet's development.If that is indeed the case, should we not further pursue this research and thereby forecast our planet's future habitability?"

Marie-Françoise Lefèvre

The atomic weights of these elements are too close to be able to differentiate between them.

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Mars - a guinea pig for the Earth

In 2013, the European Space Agency (ESA) is planning to launch the ExoMars mission, whose main aim is to determine whether the planet knew biological life at some time in the past. The landing platform will be equipped with a multitude of observation instruments, including those of the LaRa radioscience experiment, the data from which will make it possible to answer a fundamental question: does Mars have a solid or liquid core?

As the Earth's trajectory can be pinpointed in time down to the very last centimetre, scientists will use the LaRa instrument to measure, via the Doppler effect, the relative position of Mars compared with the Earth to thereby locate the platform with the same precision. This data will highlight the phenomenon known as nutation or the periodic oscillation of a planet's rotating axis. These oscillations directly depend on the differential rotation of the core compared with the mantle and thus, in general terms, on the liquid or solid state of the core.

In addition, the surface of Mars is formed by a single plate. In the absence of tectonic movement, certain surface rocks are over 4 billion years old. The planet's magnetic history is therefore preserved and is directly accessible. Scientists in charge of the mission hope that the net result of the data obtained will be a significant improvement in our knowledge of Mars' past and in particular in our understanding of its habitability.